1. Field of the Invention
The present invention generally relates to a congestion information setting system used in a local area network (LAN). More specifically, the present invention is directed to such a technique capable of transferring congestion information in a transmission service for connecting a frame relay network to an ATM network.
2. Description of the Related Art
A transmission service capable of realizing an interworking function by connecting an FRS (Frame Relay Service) network to an ATM (Asynchronous Transfer Mode) network is known in the technical field.
FIG. 2 schematically indicates a structure of a system capable of realizing this interworking function service. In this figure, reference numeral 201 shows an ATM network, reference numeral 202 indicates an FRS/ATM converting function unit, reference numeral 203 denotes a node, and reference numeral 204 represents a frame relay terminal connected to this node 203. In this system, both the node 203 and the frame relay terminal 204 constitute a frame relay network 205.
In such a system arrangement, a frame relay format is converted into an ATM cell format, and also an ATM cell format is converted into a frame relay format in the FRS/ATM converting function unit 202. That is to say, a format conversion in a direction from a frame relay format to an ATM cell format is indicated in FIG. 3. Referring now to FIG. 3, a description will be made of a format converting sequence in the direction from the frame relay format to the ATM cell format.
First, in an FR-SSCS (Frame Relaying—Service Specific Convergence Sublayer) layer, both an address field of Q. 922-DLL-PDU (Dynamic Link Library—Protocol Data Unit) and an information field thereof, from which “0” bit has been deleted, are derived so as to be converted into an FR-SSCS-PDU (Frame Relaying—Service Specific Convergence Sublayer—Protocol Data Unit).
In a CPAAL5 (Common Part ATM Adaptation Layer 5), the converted FR-SSCS-PDU is acquired as a CPAAL5-SDU (Service Data Unit). A CPAAL5-PDU (Protocol Data Unit) trailer portion is added to this acquired CPAAL5-SDU, and then the resulting PDU is converted into a CPAAL5-PDU.
Furthermore, this CPAAL5-PDU is divided every 48 octets into SAR-PDU.
In an ATM layer, an ATM header containing a PTI and the like is added to this divided SAR-PDU (Segment And Reassembly—Protocol Data Unit), and then the resulting PDU is converted into an ATM cell which is transmitted to an ATM-UNI (Asynchronous Transfer Mode—User—Network Interface).
On the other hand, a format converting sequence along a direction from an ATM cell format to a frame relay format is carried out opposite to the above-explained format converting sequence.
In the transmission service for providing such an interworking function, a congestion control was carried out as follows:
First, in the case of a forward direction from a frame relay format to an ATM cell format, an FECN (Forward Explicit Congestion Notification) indicative of congestion information of Q. 922-DLL is not mapped to an EFCI (Explicit Forward Congestion Indication) representative of congestion information, which is stored into a PTI (Payload Type Identifier) field of an ATM cell belonging to this frame. In other words, the EFCI of the ATM cell would be always set to “no congestion”.
Next, in the case of a forward direction from an ATM cell format to a frame relay format, only a portion of congestion information in an ATM network could be utilized for a frame relay. In other words, only when an EFCI field is set as “congestion occurs” in a last received ATM cell, this congestion information may be reflected to the frame relay.
On the other hand, in the case of a backward direction from an ATM cell format to a frame relay format, a BCI (Backward Congestion Indication) is provided only at a frame level having a BECN (Backward Explicit Congestion Notification). In this case, a BECN field of an FR-SSCS-PDU is duplicated without any change to a BECN field of a Q. 922 core frame.
Also, in the case of a backward direction from a frame relay format to an ATM cell, when a BECN of a Q. 922-DLL frame transmitted along the direction from the frame relay format to the ATM cell is set as “congestion occurs”, or when an EFCI of an ATM cell contained in a segment frame last received along the direction from the ATM cell format to the frame relay format is set as “congestion occurs”, a BECN field of an FR-SSCS-PDU is set as “congestion occurs”.
However, in this backward direction, the congestion information contained in the Q. 922 core frame and the congestion information contained in the FR-SSCS are not always identical to each other as to the protocol field and the function in the ATM network. As a consequence, in such a case that different types of networks are connected to each other, distortion may occur in the congestion information. As a result, although such congestion information may not be required for a certain service form, there are possibilities that extra workloads are given to an information control unit of a network so as to process this congestion information.
The present invention has been made to solve the above-explained problems, and therefore, has an object to provide a congestion information setting system capable of transmitting congestion information even in an interworking function effected between a frame relay network and an ATM network.